420 THE AMERICAN JOURNAL OF SOCIOLOGY
as before Islam? It is evident that all these important changes can be interpreted only by means of internal and external social conditions, of which military conflicts are only the violent expres- sion, and so-called political frontiers the result. A people has never been restored to its natural boundaries, any more than it has reached them during- the period of its growth. It is, in fact, impossible to determine them. When the Turkish and Mongolian domination had begun in Asia, and after the Mongols in 1258 had overthrown the caliphate, the Arabs remained only in the Semitic countries, or in those formerly made Semitic by the Phoenicians.
The three great empires Carolingian, Arab, and Byzantine represented an unstable and momentary equilibrium, like all social and organic equilibria. The very causes which favored their formation led also to their dissolution. Any one of the three disappearing, the other two had no longer any raison d'etre.
From 806, at the apogee of his power, Charlemagne deter- mined upon the division of his empire after his death. It included regions simply tributary. At this time the empire extended beyond the Pyrenees as far as the Ebro; on the west it extended along the Atlantic, the English Channel, the North Sea, the Eider, and the Baltic Sea as far as the mouth of the Vistula, whose course bounded it on the northeast; from this point it was bounded on the east by the Tisxa, and by the Narenta as far as the mouth of the latter stream on the Adriatic; on the south it included north and central Italy, and touched the Mediterranean coast of Gaul. Neither the Alps nor the Pyrenees served as its frontiers. As to rivers, it included the great fluvial basins of the Adour, the Po, the Garonne, the Loire, the Seine, the Somme, the Scheldt, the Meuse, the Rhine, the Ems, the Weser, and the Elbe, and, besides, the basin of the Danube as far as the country of the Avars. A military march was established beyond the Pyrenees, where the empire was in direct contact with the Arab power.
At his death, in accordance with the act of partition, the empire was divided among his three sons; this could be done without danger at that time, or else it is probable either that the Germanic custom had been modified in regard to the right of primogeniture, or that, in the absence of this adaptation, the
